This invention relates generally to vortex-type flowmeters acting to convert fluidic oscillations to corresponding mechanical vibrations which are transmitted to an external force sensor, and more particularly to a sensing system for a flowmeter of this type whose output signal is indicative of the mass of the fluid passing through the meter.
It is well known that under certain circumstances the presence of an obstacle in a flow conduit will give rise to periodic fluidic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed, these being known as Karman vortex streets. The periodicity at which vortices are shed in a Karman vortex street is a function of flow rate.
In many industrial processes, one must be able to measure the volumetric flow of fluids being treated or supplied in order to carry out various control functions. It is also necessary, in some instances, to determine the mass flow of the fluids. Existing types of vortex flowmeters are capable of effecting volumetric flow or mass flow measurement.
The Burgess U.S. Pat. No. 3,888,120 discloses a vortex-type flowmeter including an obstacle assembly mounted in a flow tube through which the fluid to be metered is conducted, the assembly being formed by a front section fixedly mounted across the tube and a rear section cantilevered from the front section by means of a flexible beam to define a gap serving to trap the Karman vortices. Because the rear section is deflectable, it is excited into mechanical vibration by the vortices at a rate whose frequency is proportional to fluid flow. These mechanical vibrations are sensed by a strain gauge mounted on the flexible beam, the gauge serving to convert the mechanical vibrations into a corresponding electrical signal whose frequency is a function of flow rate.
In the Herzl et al. U.S. Pat. No. 4,033,189, the transducer for converting the mechanical vibrations produced by the deflectable section of the obstacle assembly into an electrical signal is a force sensor external to the flow tube. To this end, the vibrations are mechanically transmitted from the deflectable section of the obstacle assembly to a coupling point outside the flow tube, which coupling point is engaged by the force sensor.
In many industrial processes and in various chemical applications, one must not only measure the volumetric flow of fluids being treated or supplied, but also the mass flow thereof. Thus in certain chemical applications, the quantities of reactants are best specified according to mass, and in metering combustible gas supplied to consumers, one must know the total mass of the gas supplied. When standard volumetric flowmeters are employed for this purpose, it is necessary to convert the volume measurement to a reading of mass flow.
This conversion is readily effected by multiplying the volume measurement by the density of the fluid being measured. Hence the volumetric flowmeter must include indicating means that are calibrated to account for the density factor. Since the density of a given fluid is a function of its temperature and pressure, should the meter be calibrated on the assumption that temperature and pressure is fixed at some value, the mass reading would be inaccurate by reason of inevitable fluctuations in temperature and pressure. Thus it has generally been necessary, in order to produce highly accurate mass flow readings, to determine the actual density of the fluid, rather than to assume an unchanging density value.
To simplify the measurement of mass flow, the Herzl U.S. Pat. No. 3,776,033 provides a vortex meter which includes a pressure-responsive transducer adapted to generate an electrical signal whose frequency is proportional to the fluidic pulse rate and whose amplitude is a function of the kinetic energy contained in the vortex. The transducer signal is processed by an operational amplifier, the gain of which is inversely proportional to frequency, thereby effectively dividing the transducer signal by frequency throughout the operating range of the meter to yield an output signal whose amplitude is indicative of mass flow. By additionally dividing this output signal by frequency with a second amplifier having a gain which is inversely proportional to frequency, a signal indicative of the fluid density is produced.
The use of force sensors such as piezoelectric pressure-responsive elements in a vortex meter has distinct advantages, for such sensors are relatively immune to dirt and other contaminating coatings and have a proven performance record. As explained in the above-cited Herzl patent relating to vortex-type mass flowmeters, pressure changes detected by the force sensor are converted in corresponding changes in amplitude in the signal generated thereby, this amplitude being a function of the kinetic energy contained in the fluid vortex. This relationship is expressed by the following equation: EQU A=KpV.sup.2
where A is the output amplitude
p is the fluid density PA1 V is the fluid velocity.
If, for example, a vortex flowmeter were to operate over a 15 to 1 velocity range and a 2 to 1 density range, the ratio signal amplitudes would equal: EQU K.sub.p V.sup.2 /K.sub.p V.sup.2 =2.times.225/1.times.1 =450 to 1
Assuming that one wished to maintain a 10 to 1 signal-to-noise ratio at the minimum condition, the sensing system for this purpose would require a signal-to-noise ratio of 10.times.450 or 4500 to 1. There are very few commercially-available sensors that would satisfy this requirement. And if one wished to operate in a 30 to 1 range with a 2 to 1 density change, this would call for a sensing system with an 18,000 to 1 signal-to-noise ratio. This broad dynamic range is virtually impossible to attain in a practical sensing system.
Another factor that one must take into account in connection with sensing systems for vortex meters is the adverse effect of temperature on the accuracy of the system. Where the sensor is disposed within the flow tube and the meter is to be used for measuring fluids in a broad temperature range whose lower end is well below 0.degree. C. and whose upper end is well above 100.degree. C., the sensor will be heated or cooled by the fluid and will produce an output having an error component reflecting the fluid temperature. Even when the sensor is of the external type, it will still be affected by the temperature of the fluid conducted through the flow tube, for the sensor is in close proximity to the wall of the flow tube.
The concern of the present invention is with sensing systems for vortex flowmeters of the external sensor type. Such sensing systems of the type heretofore known have three drawbacks:
(1) for purposes of providing a mass flow reading, they require relatively complex electronic circuits operating in conjunction with the sensing system to derive from the amplitude and frequency of the sensor signal a signal indicative of mass flow.
(2) The accuracy and reliability of the sensing system is adversely affected by the temperature of the fluid being metered.
(3) The sensor is incapable of coping with the broad dynamic range of the system.